SAN FRANCISCO, CA - SEPTEMBER 05: General view of AT&T Park with a rainbow in the background during the first inning between the San Francisco Giants and the Arizona Diamondbacks on September 5, 2012 in San Francisco, California. (Photo by Jason O. Watson/Getty Images)

Photo: Jason O. Watson, Getty Images

SAN FRANCISCO, CA - SEPTEMBER 05: General view of AT&T Park...

Image 9 of 41

Mike Pechner, a forecaster with Golden West Meteorology, took this photograph of the rainbow at the Giants game.

Double rainbow over the Financial District in San Francisco. Commented photographer Phil Dokas: "Working late isn’t so bad when this is the first thing you see when you leave the building."

Photo: Courtesy Phil Dokas

Double rainbow over the Financial District in San Francisco....

Image 15 of 41

Rainbow over the Ferry Building in San Francisco.

Photo: Courtesy Of Missy Buchanan

Rainbow over the Ferry Building in San Francisco.

Image 16 of 41

Composite photo of full rainbow over the Mission at 20th and Valencia.

Photo: Courtesy Deejay Kramer

Composite photo of full rainbow over the Mission at 20th and Valencia.

Image 17 of 41

Rainbow over San Francisco.

Photo: Courtesy Bob Gunderson

Rainbow over San Francisco.

Image 18 of 41

Rainbow over SOMA in San Francisco. Courtesy Jason Bull

Photo: Jason Bull

Rainbow over SOMA in San Francisco. Courtesy Jason Bull

Image 19 of 41

Rainbow over Mission.

Photo: Courtesy Milton Repreza

Rainbow over Mission.

Image 20 of 41

Double rainbow over San Francisco, Sept. 5, 2012

Photo: Toby Harriman, Courtesy Of Toby Harriman

Double rainbow over San Francisco, Sept. 5, 2012

Image 21 of 41

Rainbow over the Port of San Francisco.

Photo: Courtesy Jason Leder

Rainbow over the Port of San Francisco.

Image 22 of 41

A double rainbow appears over a building on Polk Street in San Francisco, CA on Sept. 5, 2012.

Photo: Cammie Toloui, Courtesy Of Cammie Toloui

A double rainbow appears over a building on Polk Street in San...

Image 23 of 41

Rainbow over San Francisco.

Photo: Courtesy John Arns

Rainbow over San Francisco.

Image 24 of 41

Call it a package deal - a FedEx plane appearing to zoom through the rainbow.

Photo: Carlos Avila Gonzalez, The Chronicle

Call it a package deal - a FedEx plane appearing to zoom through...

Image 25 of 41

Rainbow over San Francisco.

Photo: Courtesy Jimmy Zhen Wu

Rainbow over San Francisco.

Image 26 of 41

Rainbow at sunset.

Photo: Courtesy Of Gurpreet Singh

Rainbow at sunset.

Image 27 of 41

Sunset rainbow. Instagram photo.

Photo: Courtesy Of Sonia Savio

Sunset rainbow. Instagram photo.

Image 28 of 41

San Francisco Giants' Marco Scutaro, right, walks back to the dugout after an at-bat against the Arizona Diamondbacks as a rainbow appears in the sky during the first inning of a baseball game on Wednesday, Sept. 5, 2012, in San Francisco.

Thanks to a tiny but consistent warming of the Pacific Ocean half a world away, the Bay Area will experience an El Niño season this year, beginning this month.

If that idea conjures up images of fearsome storms and tumultuous winds - as one infamous El Niño pattern caused in the late 1990s - fear not. At least, possibly not.

The National Weather Service has identified a "weak" El Niño this year, as opposed to the "moderate" or "strong" varieties, suggesting that the weather pattern caused by El Niño won't last the entire winter.

But what will the weather actually be as a result of El Niño?

"It's, like, roll the dice," said Jan Null, a former lead forecaster for the National Weather Service and now an adjunct professor of meteorology at San Francisco State University. "Farmers will probably say it'll be wetter than normal, but there's no way to know."

What meteorologists can say for sure is that "El Niño " means the temperature of the Pacific Ocean near the equator rose by at least half a degree Celsius above normal for a three-month period in a given year.

This year, the temperature rose by seven-tenths of a degree, a "weak" amount. A greater rise leads to stronger El Niño patterns.

Thirsty grape vines

"In the past, El Niño has provided us with heavier rainfall," he said. "More water is better."

Vintners think in two-year planting cycles, so a year of lighter rainfall will eventually push the harvest back too far, to late fall.

Bressler said, "Last year, the joke was, 'What are you going to be for Halloween?' Answer: Picking.

"So El Niño is delightful," he said. "Especially since for the last two or three years, we've had its evil twin, La Niña."

The little girl is simply the opposite of the little boy: a slight but consistent cooling of the Pacific around the equator that may - or may not - result in more rain.

"We don't experience any difference between the two," said Tyrone Jue, spokesman for the San Francisco Public Utilities Commission, which operates three main watersheds that provide water for Bay Area households: Hetch Hetchy Reservoir in Yosemite National Park, the Alameda Watershed, and Crystal Springs Reservoir in San Mateo County.

"And as far as El Niño being weak or strong, you're in a wait-and-see mode" about predicting how much water will be collected, Jue said. "You still have to wait till the rains actually come."

General patterns

Despite the unpredictability in any given year, there are some general patterns to the El Niño phenomenon, said Logan Johnson, warning coordination meteorologist with the National Weather Service.

"When there's a weak El Niño, many times it's drier than normal - at least, it has been on average over the last 50 years," Johnson said.

That's how long meteorologists have been tracking the condition that causes different weather patterns from region to region.

What's happening with El Niño is that the jet stream - a sort of river in the atmosphere - gets pushed to the south, he said. If the jet stream is north of the Bay Area to begin with, then settles over the region, "then we'll have the storms over us," Johnson said.

Pretty scenes

Meanwhile, the Bay Area saw two other bits of wondrous weather this week.

A glorious double-rainbow settled briefly over San Francisco on Wednesday, causing thousands of people to pause and enjoy the spectacle, many capturing it with their cell phone cameras.

At AT&T Park, fans took their eyes off the Giants game to gawk at colorful arches that formed over center field, perfectly framed by the grandstands.

A double rainbow occurs when the sun is bright enough to be reflected not once, as is typical, but twice inside each raindrop, said Null, the meteorologist.

Also on Wednesday, just as the sun was setting, a fine mist mixed with golden light from a low-slung sun to cast a pink glaze across downtown skyscrapers in the city.

"We get our best sunsets with some clouds, and when it's clear down to the horizon for the sun to shine through," Null said.

That lets the color reflect off of the clouds, he said, "instead of going off into space."